Super-powered silk
Researchers at HKUST have used modified bacteria to produce spider silk with new properties that could be useful in regenerative medicine and beyond.
Spider silk is renowned for its strength and stability and it is even bio-compatible, meaning the body’s immune system does not react to its presence. These properties make it attractive for applications such as scaffolds for tissue regeneration.
Spider silk cannot be easily harvested from spiders as they will fight if confined in a farm. Additionally, the silk needs to be functionalised to be useful. For example to be used as a tissue scaffold, the silk must have bioactive properties that encourage cells to settle on its surface.
Now, a research team led by Dr Fei Sun from the Department of Chemical and Biological Engineering at the Hong Kong University of Science and Technology, has used modified E. coli to produce spider silk proteins with added molecules that provide new functions. The results are published in the journal Advanced Functional Materials.
Combining techniques
To make the E. coli produce spider silk, the team used a technique called recombinant DNA technology. This allows synthetic biologists to isolate and manipulate genes from different organisms and include them in other organisms. By introducing genes from spiders into E. coli, the team were able to make the bacteria act as mini protein factories for spider silk.
They then used a form of protein click chemistry to add functional molecules to the silk. Protein click chemistry encompasses a series of techniques to make very simple and efficient reactions that combine protein molecules with high yield and specificity. This means molecules connect in predictable ways under mild conditions, allowing the reactions to be efficient and easily reproducible.
The team used protein click chemistry to create two different functionalised versions of spider silk. The first version involved coating the silk with silica glass using an enzyme that produces the material in sea sponges. This created a hybrid organic-inorganic material which may have future applications in optical fibres.
The second version used cell-binding ligands to encourage cells to attach to the surface of the silk. The cells were by the ligands along an axis and were laid down in an orderly manner suitable for tissue building.
Repairing the body
This is only the first step, but there are good reasons to be excited about spider silk gaining this ability. Several materials to encourage tissue and nerve regeneration are being investigated, but many are too unstable in the body. Collagen and related polypeptides, for example, are derived from human connective tissue, so can be degraded by human enzymes.
Spider silk is not derived from anything the body usually comes across, like collagen or proteins from pathogens, so the body has no ‘immune memory’ for it, and is unlikely to react. Spider silk is also non-toxic to human cells.
E. coli-derived spider silk is not as strong as its natural counterpart, since spiders can spin vastly longer protein strands. However, the ability to functionalise the silk is more important in biomedical applications, and as Sun says, “we are not making clothes!”
Still, if it is to be useful in biomedical settings, a way to easily manufacture the silk will be needed, and Sun says there are companies that could work on that side of things. Meanwhile, he and the team will work to improve and expand on the functionalisation, such as adding growth factors to the cell-binding ligand system to promote cell growth on the silk.
Another related area is to promote the regeneration of axons: neuron fibres that are crucial for the function of the nervous system. For injuries to the central nervous system, such as the spinal cord or optical nerve, there is currently no cure, and researchers have long sought ways to regenerate axons in these situations.
Sun said: “If we could facilitate the regeneration of the central nervous system, we could contribute to this complex field and give neuroscientists more options.”
AI enhancement
The possibilities for functionalising silk are promising, but it’s not simply a case of knowing what you want and making it. However, this possibility is a lot closer to that than it used to be, thanks to artificial intelligence (AI). Attaching proteins together can cause unexpected effects because of the way they interact and fold: they can clash or block each other, not working as intended.
This has made a lot of protein engineering trail and error, and though skilled engineers build up good instincts, these are still essentially educated guesses. But the AI platform AlphaFold has the ability to model how proteins fold in 3D. With this, the Sun and the team can model the best place on the silk protein to attach a SpyTag to get the best effect.
“Before AlphaFold, we could only make an educated guess,” said Sun. “Now the AI tells us what might work best, and the results in the lab agree, saving time we would have spent making proteins that didn’t work. It’s changed the way protein engineers work.”
The ability to produce functionalised silk without spiders, combined with the ability to predict protein structures, opens up the possibility for creating on-demand silks based on the desired application. It’s a futuristic notion but Sun sees it as continuing a tradition starting more than 5,000 years ago in China with the harvesting of silk from silkworms.
He said: “Nature is amazing. Through evolution it has created all kinds of protein-based materials that we can tap into. There’s lots still to be studied that could open up whole new possibilities.”